In remote measurement, the main body of a measuring apparatus and an element to be measured are normally connected by cables, which can be a source of errors. FIG. 9 shows an example of a test for temperature characteristics of an element to be measured. An element to be measured 97 is placed in a chamber 92. The main body 91 of a measuring apparatus (outside the chamber) and the element to be measured are connected by a single heat resisting cable 93.
In the measurement, a raw measured value from the measuring apparatus is a value obtained from the element to be measured and includes series inductance L and parallel capacitance C of the cable. This raw measured value is corrected by eliminating the series inductance and the parallel capacitance of the cable to obtain the desired impedance of the element to be measured. However, if the series inductance and the parallel capacitance of the cable vary due to temperature or mechanical stresses such as bending of the cable, an error can occur in the impedance value of the element to be measured.
A simplified example will be presented in the case of a measurement performed on a capacitor having high impedance wherein the series inductance of the cable can be neglected. As shown in FIG. 10, it is assumed that the cable, an element to be measured A and an element to be measured B have respective capacitances (Cx) of 100 pF, 1000 pF, and 10 pF. The measuring apparatus measures 1100 pF and 110 pF from elements A and B, respectively, and performs correction by subtracting the capacitance of 100 pF of the cable therefrom to obtain correct values of 1000 pF and 10 pF. Assume that the capacitance of the cable is increased by 1%, i.e., 1 pf from the value which has been measured in advance, as a result of a change in temperature, bending of the cable and the like. Then, the measured values which have been corrected will be 1001 pF for element A and 11 pF for element B. The error is only 0.1% for the former but is as large as 10% for the latter. In other words, the greater the impedance (the smaller the capacitance) of an element to be measured, the more serious the effect of the parallel capacitance of the cable on measurement errors.
Similarly, the smaller the impedance of an element to be measured, the more serious the effect of the series inductance of the cable. This is summarized in FIG. 11 which shows the tendency towards error relative to the impedance Zx of an element to be measured.
There is a method for preventing reduction in the accuracy of impedance measurement in low and high ranges due to changes in the characteristics of a cable, wherein the voltage across an element to be measured and the current flowing therethrough are separately measured using two or more cables and are compared to obtain the impedance of the element to be measured. The four terminal pair measurement is typical of this approach. The two-terminal-trio method (refer to Japanese Patent Application No. H03-274543) is suitable for high frequencies wherein grounding of an element to be measured is possible. In either method, the overall effect is smaller, the higher the frequency. In addition, a negative feedback amplifier for maintaining the current or voltage of an element to be measured at a certain value must be used in both of these methods. Such a negative feedback amplifier operating at high frequencies is complicated and is difficult to implement in practice.
Under such circumstances, the volt-ammeter method (hereinafter referred to as V-I method), wherein no negative feedback amplifier is used, has been devised and put in use to cover high frequency bands ranging from 1 MHz to 1 GHz. One version of this approach is to use a current transformer. In another approach, the current transformer is replaced by a balun to deal with wider bands, with impedance matching taken into consideration. Such versions of the V-I method, however, use a transformer or balun in the vicinity of a measurement terminal and, therefore, are not appropriate for the purpose of evaluating temperature characteristics of an element to be measured. The reason for this is that since such a portion is left in a chamber, the measuring apparatus is adversely affected by temperature. Usage is absolutely limited by the Curie-point of magnetic elements when the temperature slightly exceeds 100.degree. C.